Cable Cars
in
The Sky
Hans Moravec
Artificial Intelligence Lab
Computer Science Dept.
Stanford University
Stanford, Ca. 94305
Copyright 1978 by Hans P. Moravec
All Rights Reserved
Introduction
Once upon a time, long before people could fly, a foolhardy
few reached for the heavens with mountaineering and masonry. Mystics
scaled peaks to commune with the gods and monarchs commissioned huge
civil engineering projects to push penthouses into the sky. But their
technology was modest and their successes minor. To this day, ridicule
is their chief reward.
By Newton's time humanity's bag of tricks had grown, and
included powerful mass-launching weapons. Armed with these, and more
importantly with his new mechanics and gravitational theory, Newton
contemplated artificial satellites orbited by a big cannon on a lofty
peak. The concept illustrated the relation between falling apples and
the unfalling moon, but the equations showed no peak was high enough,
nor cannon then powerful enough, to actually do it.
The improved cannon of the nineteenth century, though still
inadequate, were the most promising method for entering space. During
World War I projectiles from long range guns grazed the top of the
atmosphere.
Rockets, cardboard or metal tubes stuffed with gunpowder, had
been used for entertainment and war for centuries, but poor
performance limited their utility until World War II. Calculations by
the early space theorists showed that a rocket could, in principle,
propel objects away from Earth more gently than a cannon. The required
size was huge, and grew exponentially as the energy content of the
propellant fell. Gunpowder, for instance, was terrible. To boost a ton
to escape velocity a black powder rocket needs an initial weight of
200 million tons. This compares to a mere 15 tons for a rocket
burning hydrogen and oxygen, one of the most energetic chemical
combinations.
Of late rockets have been so successful that the other roads
to the sky have been nearly forgotten. Even exotic proposals such as
nuclear pulse and laser propulsion are variations on the rocket
principle. It is nevertheless true that the limitations on towers,
cannons and rockets are about the same. All methods limited by
chemical bond energies (including combustion, light, electricity,
magnetism and passive structures) must be pushed to extremes to lift
objects out of Earth's gravity well.
The alternatives have been considered for the easier tasks of
leaving the moon and moving in free space. The lunar mass driver in
O'Neill's space colonization plans is a magnetic cannon, while
tethered satellites, such as NASA will dangle from the space shuttle
to observe the ionosphere, and (vaguely) solar sails are relatives of
the tower philosophy.
If the techonology is pushed to its theoretical limits, the
other methods will also work for Earth. Recent calculations by Rod
Hyde and Lowell Wood of Livermore, and independently Henry Kolm of
MIT, indicate that a terrestrial electromagnetic mass driver can send
objects into the solar system through the atmosphere. The mass driver
must boost payloads to somewhat beyond Earth escape velocity. The
ablation-shielded capsules would plow through a kilogram of air for
each square centimeter in their frontal surface area.
This article is about developments on the Tower of Babel
front. The structures discussed are not made of brick, stone or
steel, for that would be like powering interplanetary rockets with
gunpowder. They use the strongest and lightest materials available.
While all the proposals involve cables under tension, the more
traditional compressional towers are also possible, in principle.
The technology of super strength structures is less mature
than that of ultra energetic chemical reactions. Hydrogen/oxygen
combustion, which powers the space shuttle, is nearly the most
powerful chemical reaction known. Meanwhile the strongest materials
produced in quantity are twenty times weaker than the theoretical
limits, though tiny samples of substances with half the ultimate
strengths have been synthesized in laboratories.
Crystalline graphite is the hydrogen/oxygen of materials.
It's theoretically twenty times as strong as conventional steel and
four times less dense, making it 80 times as good.
Graphite fibers embedded in epoxy binders are aerospace's new
wonder materials. The space shuttle main engine owes its
unprecedented power to weight ratio to their use in the combustion
chamber and nozzle. Though strong, they are ten times weaker than
theory predicts. The reasons for this should become better understood
in the years to come. A further factor of two or three improvement
will make them adequate for terrestrial orbital towers. They are
already more than strong enough for such towers (the tensile variety
of which I will call skyhooks) on the moon and in free space.
Graphite composites are being challenged by a new class of
synthetics, the aramid fibers, introduced by the DuPont company.
Kevlar, a prototype of the class, is as strong as most existing
composites. The carbon backbone of the polymer molecules bears the
load and future aramids will likely be even stronger. Kevlar is being
used in large quantities for bullet proof clothes, radial tires,
parachutes and other applications previously served by nylon. Its
strength, durability, availability and relatively low cost make it a
prime candidate for near term skyhook projects.
Synchronous Skyhooks
On Earth
This is the classical Tower of Babel approach, updated for
sixteenth and seventeenth century discoveries in physics and
astronomy. In "Speculations between Earth and Sky", written early
this century, Konstantin Tsiolkovsky discussed a hypothetical tower
built from the equator to tremendous heights. A person climbing the
structure would experience decreasing gravity (and air) with
increasing altitude. 36,000 kilometers above the ground the
centrifugal force of the earth's rotation equals the lessened gravity,
and it becomes possible to let go of the tower and float freely beside
it. Beyond this synchronous point the centrifugal force dominates,
and the climber is pulled outwards, away from the earth. To
Tsiolkovsky it was a mental exercise, to illustrate concepts in
celestial mechanics. Actual construction was dismissed as an obvious
impossibility with known technology. The tallest artificial structure
of the day, the Eiffel Tower, was 300 meters high.
Discussions of the concept have traditionally centered on its
self evident absurdity. There are problems in physics texts which ask
the reader to show how a tower built up to the sky, or a cable
dangling down from it, could not possibly support its own
weight. Similar problems demonstrate the inanity of interstellar
flight, and earlier textbooks disproved the possibility of journeys to
the moon, and of heavier than air flying machines.
Although a compressional tower may be possible using some
active means to prevent buckling, a cable under tension is a simpler
structure. In 1960 another Russian, a young Leningrad engineer named
Y. N. Artsutanov, published calculations about a cable grown from a
synchronous satellite. One end would be extended towards the earth,
and would weigh the satellite down. The other end would be extruded
upwards, lifting the satellite by virtue of the orbital centrifugal
force. If the extrusion rates of the two were carefully controlled
the net pull could be kept nil. Eventually the lower end would reach
the ground. At that time the outer end of the cable would be 150,000
kilometers away from Earth. Tidal force would keep the structure
stretched and vertical, and it would hover just above the surface, in
perfect equilibrium. The bottom end could then be anchored to the
ground, and a large counterweight attached to outer tip. Being far
beyond synchronous orbit, this counterweight would pull on the cable,
and thus on the anchor.
The cable, under full tension, can now support elevator cabs
running up and down its length. At synchronous height they may let go
to become synchronous satellites. The energy for achieving the orbit
comes partly from the long climb, but also from Earth's rotational
energy, which accelerates payloads to orbital velocity by a small
deflection of the cable.
Cabs continuing beyond the synchronous point are pulled along
by the ever increasing centrifugal force. They can extract energy from
the ride, and on reaching the ballast can have recovered the energy of
the climb to synchronous altitude. Such full route passengers are
powered entirely by Earth's angular momentum. The ballast is 150,000
kilometers from the center of the earth and moving with a velocity of
11 Km/sec. A cab released from there has enough momentum to coast to
the orbit of Saturn on a Hohmann minimum energy trajectory.
No known substance can support its own weight from synchronous
orbit if it's simply fashioned into a uniform rope. To make the
concept barely feasible the cable must be trimmed of all excess mass.
At each height the cross section should be just large enough to
support the local tension. The skyhook takes on the form of a
distorted bell curve, narrow at the ends, tapering to maximum area at
synchronous altitude. The taper (the ratio in cross sectional area
between the middle and the ends) is exponential in the weight to
strength ratio of the material, which means that if you halved the
strength or doubled the density the taper would square. If it was 10
before, it would be 100 after, if it was originally 100 it would
become 10,000.
The skyhooks' size, and very feasibility, is extremely
sensitive to the strength of the material one proposes to use. A
synchronous cable made of steel has to be 10^50 times bigger in the
middle than at the ends and weigh 10^52 times what it can
support. Astronomical results like these are the basis of physics text
skyhook impossibility proofs. Rockets are similarly sensitive to the
energy content of their fuel, and open to the same kind of ridicule.
A coal fired steam rocket needs to weigh a 100 billion times as much
as its payload, to achieve escape velocity.
Per unit weight, Kevlar is 5 times stronger than steel. A
synchronous Earth skyhook made of it needs a taper of 10^10 (ten
billion), and weighs 10^13 times what it can lift. Better than steel,
but still ridiculous.
Single crystal graphite whiskers with 50 times the strength to
weight of steel have been grown in laboratories. Bulk material as
strong would permit a synchronous cable with a taper of only 10, and a
mass ratio of 400. Half this strength is perfectly adequate for an
Earth synchronous skyhook.
Artsutanov's results were printed in popular Russian
literature, and showed up in the US in several Soviet published
English language books and magazines, where they were widely ignored.
fig 1: A terrestrial synchronous graphite skyhook. The diagram is to
scale, except that the thickness of the cable has been greatly
magnified.
In 1966 John Isaacs, Allyn Vine, Hugh Bradner and George
Bachus from the Scripps and Woods Hole Institutes of Oceanography
independently derived the properties of an Earth skyhook. They
managed to get their results published in Science, along with a note
from the editors effectively apologizing for printing such a grandiose
and futuristic idea. Their paper elicited a letter from an officer of
the Novosti Press agency pointing to and claiming credit for
Artsutanov's earlier work.
The time for skyhooks was still not ripe. The idea was again
forgotten, and no new work appeared for a decade.
In 1975 Jerome Pearson, an engineer working at the Flight
Dynamics Laboratory of Wright Patterson Air Force Base, again
independently derived and published the concept, this time in Acta
Astronautica, an international astronautics journal. Undaunted on
discovering the idea was already in the literature, he published a
second paper in 1976 outlining an operational mode for the Earth
skyhook, analyzing its dynamic response to moving payloads. He found
that certain elevator velocities excited resonances in the cable, but
that a cab could safely accelerate through these speeds, and that the
system was generally workable.
On The Moon
In 1977 and 1978 Pearson published papers about synchronous
skyhooks on the moon. At first glance a lunar skyhook seems even more
absurd than a terrestrial one. The moon rotates very slowly, once a
month, and synchronous altitude is 400,000 kilometers, the Moon-Earth
distance, above the lunar surface. A Moon skyhook apparently has to be
much bigger than an Earth model.
Fortunately the Earth-Moon interactions come to our aid. In
1772 comte Joseph Louis Lagrange, a French mathematician, tackled the
interactions of two heavy masses and a very light one in Newton's
newfangled physics. There are five places near two heavy bodies in
circular orbits about each other where gravitation and the orbital
centrifugal force cancel. A small object with the right velocity in
any of these locations will whirl around with the big bodies and
appear stationary with respect to them.
In the context of the Earth-Moon system, the first point,
dubbed L1, is on the line joining the center of the earth to the
center of the moon, closer to the moon than to the earth. At L1
centrifugal force and lunar gravity team up to cancel the earth's
pull. L2 is behind the moon, where the centrifugal force cancels the
combined pull of the earth and moon. L3 is similar, but behind the
earth. L4 and L5, well known to future space colonists, form the third
points of two equilateral triangles whose other vertices are the
centers of the earth and moon. The gravitational and centrifugal
forces at L4 and L5 point in different directions.
The Lagrangian points in a two body system are analogous to
the synchronous orbital altitude of a single body. L1, L2 and L3 are
unstable, which means that any positioning error will cause a
satellite to drift away. Synchronous satellites of single bodies have
the same problem, requiring continual corrections by tiny thrusters to
stay put. L4 and L5, on the other hand, are stable. They are actually
the centers of shallow, disembodied potential wells, and mass placed
there will remain.
Skyhooks anchored to the lunar surface can be built passing
through, and with maximum thickness at, the L1 and L2 points. Pearson
calculated that a lunar skyhook through L1 would be twice as long,
300,000 kilometers, as an Earth hook. The lunar gravity well is very
shallow and far less demanding of skyhook construction materials.
Existing substances such as Kevlar and graphite composites are strong
enough, and result in mass ratios (ratio of skyhook mass to maximum
payload mass) of a few hundred. An L2 skyhook is 550,000 kilometers
long, and twice as heavy as a similar L1 hook.
If the the moon didn't block Earth's view of L2, a
communications satellite there could link Earth and the lunar farside.
The idea has merit because it's possible to put objects into circular
orbits, called halo orbits, around the unstable Lagrangian points.
The plane of a halo orbit is perpendicular to the line joining the two
massive bodies, and from Earth an L2 halo satellite appears to be
circling the lunar disc. Recently a satellite called the
international Sun-Earth explorer was put into a halo orbit around the
Sun-Earth L1 point to observe the solar wind. While L1 itself is
visible from Earth, it is obscured by radio noise from the solar disc
behind. Halo satellites must use fuel to counteract drifting.
Pearson proposes anchoring a lunar farside satellite to the
moon's surface with a very thin, truncated, L2 skyhook. The
satellite, a little beyond L2, would not need to station keep.
Needing no fuel, its lifetime could be indefinite.
The closer to L2 the satellite is, the smaller are the
restraining forces needed to keep it there, and the more delicate can
be the tether that holds it. The tether mass can be made arbitrarily
smaller than the mass of the satellite it restrains. A limitation is
the extreme thinness of the lower end of the skyhook. Suppose we are
building with a graphite composite material that needs a taper of 30:1
between L2 and the ground, and that a 4000 Kg satellite is being
restrained by a 100 Kg tether. The 100 Kg is distributed over 70,000
kilometers, and most of it is near L2. The tether diameter at the
ground is then only one hundredth of a millimeter, very tiny indeed.
Of course a tethered satellite sticking out of the middle of
the lunar farside can't be seen from Earth. If we give it a little
sideways kick it will begin swinging like a giant pendulum. The plane
of the oscillation will precess, and from Earth the satellite will
seem to trace out a complicated lissajous pattern in the vicinity of
the moon, being visible virtually all the time.
On Mars
Foremost among the planets, Mars seems to have been designed
with a synchronous skyhook in mind. It has a gravity well just deep
enough to make a conventional matter skyhook interesting, a simple
gravitational environment, and a high rotation to keep the hook short.
Kevlar is almost strong enough for the job. A Martian Kevlar skyhook
would have a taper of 15,000 and a mass ratio of a million. A material
twice as strong would give a taper of 100 and a more reasonable mass
ratio of 6000. Some graphite composites occasionally achieve that.
Arthur Clarke has suggested that Deimos, 3000 kilometers above
synchronous orbit, is in exactly the right place to provide a mass
anchor for a truncated Martian skyhook. Using it would permit a
skyhook with one third the mass of an equivalent full length
cable. The dynamics are similar to Pearson's tethered lunar satellite.
Non-Synchronous Skyhooks
The Stanford Artificial Intelligence Lab (we're trying to make
smarter computers) is frequently a hotbed of extreme, usually
technical, ideas. Among the extremists is John McCarthy, the lab's
founder. In the 1950's he conceived a series of ideas about space
travel.
A synchronous Earth skyhook was among them. Looking up the
strongest material to be found in the CRC Handbook, which in those
days was steel, he did a rough calculation on the required taper and
got the 10^50 figure that has probably nipped hundreds of potential
skyhook theorists in the bud. Chagrinned, he tried to think of cheaper
variants.
One involved starting with a satellite in a lower than
synchronous orbit. Earth's orbital velocity is extremely high, so
simply dangling a cable from a low orbit to the ground doesn't
work. But if the satellite with two cables (for balance) spins so that
the rotation cancels the orbital velocity when the tips get near the
ground, the worst effects disappear. The cables then move like two
spokes of a huge wheel rolling on the surface. The concept trades the
extreme size of a synchronous skyhook for high spin induced
centrifugal forces. While it didn't seem to offer economies dramatic
enough to offset the 10^50 figure, it was a cute idea, and McCarthy
told it to whoever seemed receptive, including me.
My late edition CRC Handbook had a new entry, a NASA table
listing the mechanical properties of single crystal whiskers of
various substances. Graphite whiskers have a much higher strength to
weight than steel, and a little effort seemed worthwhile. Using
Macsyma, a huge and very clever computer program written at MIT that
does for algebra and calculus what desk calculators do for arithmetic,
I derived formulas for synchronous skyhooks, and plugged in the
strength of graphite whiskers. A graphite Earth synchronous skyhook
could be built with a taper of only 100, and is able to support
payloads 1/6000 as massive as itself. Amazing.
Even more amazing, a few weeks later I stumbled across
Pearson's original 1975 paper, hot off the presses, containing the
same results, and then some. It encouraged me to work on the more
complicated rolling skyhook.
fig 2: A non-synchronous skyhook's progress around a planet: two
spokes of a giant wheel.
A non-synchronous (rolling) skyhook isn't able to tap its
planet's rotational energy the way the synchronous variety does. If
it picks up a payload during a ground contact and launches it with
more than escape velocity a half rotation later, the energy must come
from the skyhook's orbital velocity and rotational momentum. It drops
closer to the ground. Conversely a skyhook gains energy when it
intercepts a speeding mass with its high velocity outer tip and lowers
it to the ground. The variations in orbital altitude make it almost
mandatory that the skyhook maintain a safe distance from the
surface. A relatively tiny vehicle would be adequate for achieving a
rendezvous with a slow moving cable tip at an altitude of fifty
kilometers.
Burke Carley of Indian Harbour Beach has written to me
suggesting a more extensive use of rockets in combination with
skyhooks. The mass ratio of each method is exponential in the
velocity change it provides. If the job of achieving orbit is broken
up equally between the rocket shuttle and a skyhook, the combined
initial mass is minimized. The mass of the rocket and of the skyhook
is about the square root of what it would have been had either been
used exclusively. The skyhook material strength requirements are
halved.
The forces on a non-synchronous hook are constantly changing.
Maximum stress happens when a cable touches down, and gravity and spin
centrifugal force work in the same direction. Making the skyhook
shorter decreases the interval over which the forces sum up, but
increases the required spin rate, and centrifugal force, in a way that
eventually offsets the decreased length. Taper and mass ratio are
roughly minimized when the radius of the skyhook is one third the
radius of its planet. Skyhooks longer or shorter than this optimum are
fatter and weigh more, for the same lifting capacity.
Hypothetical graphite whisker material that permits a 100:1
taper Earth synchronous skyhook gives us an optimum size version with
a taper of 10, weighing only 50 times as much as it can lift. If
built to support 1000 tons, it would be about as long as a
transatlantic telephone cable but smaller in cross section and
considerably lighter. It orbits once every two hours, alternate tips
touching down every 20 minutes. The huge size makes the approaches,
cusps of a cycloid, appear essentially vertical. The satellite end
descends with a 1.4 g deceleration, comes to a full stop, then ascends
again with the same acceleration. There is negligible horizontal
motion.
The moon's slow rotation does not handicap a non-synchronous
skyhook the way it does a synchronous one. An optimally sized lunar
version built of ordinary Kevlar has a taper of 4, and masses a mere
13 times its lifting capacity. The modest size makes it a fine
alternative to rockets for getting supplies and personnel to and from
the lunar surface. The proposed lunar mass driver is unsuitable for
this because of its 1000 g accelerations, small payload unit and
inability to decelerate incoming capsules.
Operating a lunar transportation system based on a rolling
skyhook will require occasional adjustments of the satellite's orbital
and rotational velocity, even if on the average the amount of mass
raised equals the amount lowered. Solar powered high specific impulse
ion rockets in the skyhook's midriff are one solution.
The lunar mass driver offers another possibility. There is an
orbit which takes a projectile launched parallel to the lunar surface
by a mass driver to a perfect, velocity matched, rendezvous with the
tip of a lunar skyhook. At the instant of rendezvous the skyhook is
almost exactly horizontal, halfway between touchdowns. If the
intercepted mass is released at the upper extreme of the tip's
trajectory, it flies away with more than escape velocity and the
skyhook loses energy. If, instead, the mass is retained until the
tip's next ground contact, then dropped, the skyhook gains. The
required precision in position, time, and velocity will necessitate
some kind of terminal guidance. Ability of the mass driver to deviate
its launch angle from the purely horizontal would also help, as would
the existence of a catcher's mitt at the skyhook's end.
Docking with a non-synchronous skyhook will be nothing at all
like connecting with an orbiting spacecraft. The ground end of an
Earth skyhook is subjected to 1.4 g of centrifugation and 1 g of
gravity, for a net acceleration of 2.4 g. A lunar skyhook grounds with
half a g. Unless the docking spacecraft has plenty of fuel to waste
chasing the receding tip, the mating will have to occur within
seconds.
The Air Force's Big Bird spy satellite occasionally ejects
film packets which decelerate and re-enter the atmosphere. As a pack
parachutes down an airplane with a big hook flies by, snags the chute
lines, and snatches it from the sky. I visualize skyhook docking
techniques as refinements of this technology, combined with the
computerized guidance methods of surface to air missiles.
fig 3: A trajectory that takes a mass launched from the lunar surface
by a mass driver to a velocity matched rendezvous with the tip of an optimum
lunar skyhook. The dotted paths mark the movement of the skyhook tips.
The dashed line is the orbit of launched mass if unintercepted.
The solid arc is the actual path from the mass driver to the skyhook.
The hook can gain or lose energy by releasing the mass later.
Free Space Skyhooks
The concept of a non-synchronous skyhook works even in the
absence of a planet. A large cable rotating in empty space can catch a
spacecraft with one of its tips, hold onto it for a certain length of
time, subjecting it all the while to centrifugal forces, and then
release it with altered velocity. Maximum velocity change, twice the
velocity of the cable tips with respect to the middle, is achieved
when the spacecraft is carried through a skyhook rotation of 180
degrees.
Planetary skyhooks need to be of a certain size and strength
just to exist in their gravity wells. Free space skyhooks are
liberated from this constraint, and can be made arbitrarily large or
small.
The cross section of a well designed free skyhook varies along
its length as a perfect normal (bell) curve. For a given material the
mass ratio is exponential in the square of the skyhook tip velocity,
and independent of its length. The skyhook can be either short and
fat and rapidly spinning, imparting high accelerations for short
periods, or very long and thin and slowly rotating, taking a long time
to deliver its momentum change. The long thin ones give more time for
docking, subject their payloads to lower accelerations, and are
probably the best kind to build first. The shorter ones can transfer
payloads more frequently, and may be useful when the solar system
traffic warrants the extra difficulty.
The extreme dependence of mass ratio on tip speed means that a
material with a given strength/weight ratio is useful only up to a
certain velocity. Kevlar can be made into skyhooks with a tip speed
of one quarter Earth escape velocity, and a mass ratio of
400. Material twice as strong would permit the same performance with a
mass ratio of 20.
Built to heft the 50 ton mass of Skylab, a quarter Earth
escape velocity Kevlar free skyhook would weigh 20,000 tons. It could
be 100 kilometers in radius, with a diameter of 6 cm at the ends and
47 cm in the middle, rotating once every 4 minutes, subjecting
payloads to 8 g of acceleration. Alternatively, with the same weight,
its radius could be 20,000 kilometers, with a rotational period of 12
hours. It would then be half a centimeter thick at the ends, three in
the middle, and the acceleration would be a puny four hundredths of a
g.
This kind of hook orbiting the sun at a distance of one AU
(same as Earth) can match the velocity of a payload coming in from the
orbit of Venus on a Hohmann minimum energy transfer and boost it into
a Hohmann orbit to Mars. Eight such skyhooks could span the solar
system. One would be needed in the orbit of Mercury, one halfway
between Mercury and Venus, one each in the orbits of Venus, Earth and
Mars, one in the asteroids, one by Jupiter, and a final one at Uranus.
Each provides enough delta v to get a payload to the next one, and
Uranus' accelerates to solar escape velocity. The trip from Mercury to
Earth takes less than a year, as does Earth to Mars. An extra year
and a half is needed to reach the asteroids, and the outer planet part
of the journey takes decades.
Such maneuvers need skyhooks in the right place at the right
time. The required precision is greatly reduced if they have a little
more than the bare minimum Hohmann velocity. There is then some
leeway in launch angle, and consequently in arrival time and position.
Having more than one skyhook in an orbit would also help.
The navigational constraints can be further relaxed, and
travel times considerably shortened, if other methods of propulsion,
like ion rockets, are used between skyhook boosts.
Outgoing Hohmann-Hohmann boosts don't affect the skyhook's
rotation, but do steal orbital momentum. Although free skyhooks have
much more leeway than their planetary relatives, the energy has to be
returned sooner or later. The most convenient source is spacecraft
moving inward in the solar system. The skyhook system could support
solar system commuter traffic at no net energy cost. Outgoing
spacecraft would borrow energy from the skyhooks, and return it when
they came back. Large, nonrotating, solar sails attached to the
skyhook middles might also be used to adjust the orbits.
The hypothetical graphite material essential for Earth
skyhooks can provide twice the velocity change of Kevlar. With it,
solar system hopping would go about twice as fast.
But, But ...
Even after the material strength objections are disposed of,
skyhooks present many targets for the nit pickers. For one, they are
BIG. The non-synchronous kinds are smaller than transatlantic phone
cables, but phone cables don't whip around at orbital velocity. Apart
from an occasional trawler or sperm whale, nothing ever runs into a
phone cable. But the space around a planet is filled with orbiting
objects, moving at relative velocities faster than speeding
bullets. If one of these collides with a skyhook, the result is a
spectacular mess.
At the point of collision the skyhook separates into two
parts. One flies off into deep space, the other crashes to the
ground. On Earth the groundward fragment would burn up in the
atmosphere like a meteor, producing a momentary sheet of flame in the
sky. A lunar skyhook would impact the ground, possibly leaving an
unusual linear crater.
The silhouette of a skyhook is long but thin, with smaller
total area than many big satellites, meaning the probability of a
collision is very low. Still, skyhooks will operate mostly in the
equatorial plane, where the heavy traffic is. The consequences of a
collision could be very serious, especially to the colliding object
and to payloads or passengers on the hook. Some kind of law of the sea
will probably have to prevail. A skyhook is not very maneuverable,
but its path can be predicted in advance. It will be the
responsibility of other traffic, guided by a latter day Norad, to not
get in the way. This seems unrealistic now, when most satellites are
out of control once in orbit, but in the near future reusable
spacecraft will dock with satellites routinely, to repair or retrieve
them, or alter their trajectories.
Free space skyhooks probably don't have to worry about traffic
density, but are in danger from the very payloads they service. A
docking gone awry can result in the spacecraft hitting the wrong
portion of the hook with meteoric velocity. Fortunately the collision
probability is low. In any case it takes the spacecraft very little
energy to change its line of flight enough to miss the slender
skyhook, a maneuver that should be planned for.
The navigational calculations needed to keep a skyhook
transportation system working are not trivial. Several big computers
will be kept busy continuously predicting the orbit and rotation of a
skyhook, taking into account such matters as skyhook stretch, payload
sequences, solar wind and aging of the structural material. In
addition they must plan skyhook dockings and energy adjustments,
including the possibility of docking failures, and perhaps keep a
lookout for collision hazards.
Skyhooks are definitely a second generation system. In San
Francisco horse drawn wagons preceded the cable cars. Analogously, in
space, rockets are blazing the trails and skyhooks will be an early
sign of dawning maturity.
References
Y. Artsutanov, V Kosmos na Elektrovoze,
Komsomolskaya Pravda, July 31, 1960
(contents described in Lvov, Science 158, p 946, November 17, 1967).
J.D. Isaacs, A.C. Vine, H. Bradner, G.E. Bachus, Satellite
Elongation into a True "Sky-Hook",
Science 151 p 682, February 11, 1966 and 152,
p 800, May 6, 1966.
J. Pearson, The Orbital Tower: A Spacecraft Launcher Using
the Earth's Rotational Energy,
Acta Astronautica 2, p 785, September/October 1975.
J. Pearson, Using The Orbital Tower to Launch
Earth Escape Payloads Daily,
27'th IAF Congress, Anaheim, Ca., October 1976. AIAA paper IAF 76-123.
J. Pearson, Anchored Lunar Satellites for Cis-Lunar Transportation
and Communication, European Conference on Space Settlements and Space
Industries, London, England, September 20, 1977. To be published in
Journal of the Astronautical Sciences.
H.P. Moravec, A Non-Synchronous Orbital Skyhook, 23rd AIAA
Meeting, The Industrialization of Space, San Francisco, Ca., October
18-20, 1977, also Journal of the Astronautical Sciences 25,
October-December, 1977.
J. Pearson, Lunar Anchored Satellite Test,
AIAA/AAS Astrodynamics Conference, Palo Alto, Ca., August 7-9, 1978,
AIAA paper 78-1427.
H.P. Moravec, Skyhook!, L5 News, August 1978.